Metal waveguides and laminated waveguides are examples of transmission lines that transport electromagnetic energy. A metal waveguide is usually constructed as a metal tube in which an electromagnetic signal wave propagates along the interior of the tube by reflecting back and forth between the walls of the waveguide. A metal waveguide can be filled either with air or dielectrics and its cross-section is generally circular or rectangular.
Metal waveguides have a critical wavelength for passage of signals within. The wavelength is determined by the geometry and the size of the waveguide. Only those signals whose wavelength is shorter than the critical wavelength can propagate in the waveguide. At high microwave frequency, particularly the millimeter-wave frequency, the metal waveguide has proven to be a transmission line with minimum signal loss.
A laminated waveguide is a derivative of the metal waveguide. Instead of using a solid metal tube, a typical laminated waveguide is composed of a dielectric substrate, a pair of main conductive layers deposited on the upper surface and the lower surface of the dielectric substrate, a plurality of through conductors such as filled via-holes extending in a thickness direction in the dielectric substrate so that the through conductors electrically connect the pair of the main conductive layers and a number of sub-conductor strip layers, which are embedded and electrically connected to the via-holes within the dielectric substrate. A laminated waveguide constructed in the said way has reasonably good transmission characteristics of a high-frequency signal and has advantages in cost of production and in ability to be integrated with circuits.